Abaixo está uma instrução que descreve uma tarefa de geração de SQL, combinada com uma entrada que fornece mais contexto sobre os esquemas de tabela disponíveis. Escreva o código SQL que responda adequadamente à solicitação
 ###Instrução: Quantos valores para joined ocorrem no Guilford College?
 ###Tabelas: CREATE TABLE girls_league_championships (girls_league_championships_id AUTO_INCREMENT PRIMARY KEY, School Year VARCHAR, Volleyball VARCHAR, Cross Country VARCHAR, Soccer VARCHAR, Tennis VARCHAR, Golf VARCHAR, Basketball VARCHAR, Swimming VARCHAR, Softball VARCHAR, Track & Field VARCHAR) CREATE TABLE nationwide_tour_wins_5 (nationwide_tour_wins_5_id AUTO_INCREMENT PRIMARY KEY, Date VARCHAR, Tournament VARCHAR, Winning score VARCHAR, Margin of victory VARCHAR, Runner(s)-up VARCHAR) CREATE TABLE current_members (current_members_id AUTO_INCREMENT PRIMARY KEY, Institution VARCHAR, Location VARCHAR, Nickname VARCHAR, Founded INT, Type VARCHAR, Enrollment INT, Football? VARCHAR, Joined INT) CREATE TABLE season_two:_1999-2000 (season_two:_1999-2000_id AUTO_INCREMENT PRIMARY KEY, Series Number INT, Season Number INT, Episode Title VARCHAR, Premiere Date VARCHAR, Production Code INT) CREATE TABLE medal_count (medal_count_id AUTO_INCREMENT PRIMARY KEY, Rank INT, Nation VARCHAR, Gold INT, Silver INT, Bronze INT, Total INT)  
SELECT COUNT Joined FROM current_members WHERE Institution = Guilford College